Optical spectroscopy is one of the most powerful and widely employed analytical techniques currently in use. There is a universal demand for a non-destructive, spectroscopic probe of samples where only microgram and nanogram quantities are available. Spectra of such samples can be recorded using any of the six major optical spectroscopic techniques. The method most commonly used to obtain spectra of physically small samples is transmission. Both external reflectance and internal reflectance have also been used for microsampling, but the former is limited to examining contaminants on reflective surfaces, and the latter has not yet been fully developed as a reliable technique.
Transmission microsampling, though applicable to a wide variety of samples, has an inherent limitation which restricts the performance of the needed accessories like microscopes, beam condensers, and diamond cells. With this technique, the sample must be optically thin. Many physically small samples are not naturally optically thin and hence must be made so prior to analysis. Samples are typically diluted or physically compressed to produce a concentration or thickness that can be examined by transmission. This process frequently destroys the sample integrity and may produce distortions in the resulting spectrum.
Internal reflectance spectrometry has no such restriction. Since the sample is simply placed in optical contact with a prism, internal reflectance requires little or no sample preparation. The pressures required to achieve good contact in internal reflectance are much lower than those needed to flatten a solid sample for transmission studies and hence would retain the integrity of the sample without special distortions. In addition, internal reflectance can be used to analyze samples that are typically difficult to analyze in transmission, such as opaque substances and films on opaque substrates.
However, to examine physically small samples in internal reflectance, the dimensions of the active portion of the internal reflection element which supports the sample needs to be comparable to the size of the sample. One such internal reflectance accessory has been designed and is currently available from Harrick Scientific Corp. See also U.S. Pat. No. 4,732475. Known as the Nanosampler, this accessory, while effective for liquids and soft surface solids, utilizes an internal reflection element that fractures easily and has a somewhat obstructed small sampling area, making it difficult to position the sample. These factors make the Nanosampler nearly impossible to use with hard samples which require compression, or on a routine basis.